#include<iostream>
#include<string>
using namespace std;
string st, ed;
void turn(int x)
{
	if (st[x] == 'o') {
		st[x] = '*';
	}
	else {
		st[x] = 'o';
	}
}
int main()
{
	cin >> st >> ed;
	int n = st.size();
	int cnt = 0;
	for (int i = 0; i < n; i++) {
		if (st[i] != ed[i]) {
			turn(i);
			turn(i + 1);
			cnt++;
		}
	}
	cout << cnt << endl;
	return 0;
}
